In the field of gas analyzing and measuring technologies, there has been an increased need for more accurate instruments and techniques that can be operated quickly and in a number of different, often difficult surroundings. One such situation where this need arises is in the field of mine safety where it is necessary to quickly and accurately monitor methane gas levels. In a mine environment, it can be appreciated that methane can occur in randomly distributed pockets that must be remotely detected and/or measured prior to human exposure to such gas. Additionally, since such detection or measurement must be accomplished in a confined area having dimensions which can vary infinitely, the instrument which accomplishes this detection or measurement must be portable and furthermore must be capable of operating over distances that can vary significantly.
Presently, for purposes of detecting the presence of methane in a mine environment, a hand held catalytic detector is typically used which must be taken up to the working face in order to detect the presence of methane. This is a dangerous and time consuming operation and it would be a great advantage if the detection operation could be performed at a distance away from the coal face.
There are other existing gas detection and/or measurement techniques that can effectively recognize methane, however, such techniques accomplish this task in a manner that does not lend itself to remote, portable instrument applications. For instance, if a solid electrolyte sensor were used, it would be necessary to provide a sample of methane gas as a reference to be introduced to the reference cell side of the sensor. U.S. Pat. No. 3,915,830 which issued to A. O. Isenberg on Oct. 28, 1975, discloses that when the gas environment under study is exposed to the sensing electrode of the cell, an EMF signal is generated which corresponds to the difference and partial pressure in the gas species across the electrolyte.
To utilize this technology in a mine environment or any other environment which would require quick, accurate measurements in a large number of randomly sized areas, would exceed the practical capabilities of this technology. It is known that such sensors are used in a probe type of device which must be placed in proximity to the gas of interest for a specific period of time in order to achieve a sufficient reading. Accordingly, an application which requires essentially a scanning operation over the area in question would be impractical for this technology.
Another technology which has proven promising in the area of gas analysis and measurement utilizes the measurements of the optical absorption properties of the particular gas to detect and or quantify such gas. This technique takes advantage of the fact that, at specific light wavelengths, certain gases exhibit specific absorption characteristics. An example of the use of spectrographic techniques for gas detection can be found in a device known as an acousto optic tunable filter, commonly known as an AOTF. U.S. Pat. No. 3,792,287 issued to G. W. Roland et al. on Feb. 12, 1975 discloses the use of a Thallium Arsenic Selenide (TAS) crystal which has the property that, with infrared light applied in one direction to the crystal and an RF signal applied in another direction to the crystal so as to intercept the infrared light signal, based on the geometry of the crystal, there is formed thereby, a specific absorption bandwidth by which the detection of the gas having absorption properties coinciding with this bandwidth can be detected and/or quantified. Although this approach has proven effective for a number of industrial environments such as in a combustion control process, it also does not lend itself to an application in an environment where it is necessary to scan areas of unknown size and composition to detect pockets of the gas of interest.
Still another technology used in the area of gas analysis and measurement is that of differential absorption spectroscopy where a dispersive device such as a diffraction grating can be utilized to tune to an absorption line associated with the gas of interest and a transmission line which is off of the absorption line associated with the gas of interest, an example of the use of this technology can be found in U.S. Pat. No. 3,939,348 which issued to J. J. Barrett on Feb. 17, 1976. In this patent, a Fabry-Perot Interferometer is used to provide a plurality of transmission windows regularly spaced in frequency. Selectively separated periodic spectra which are made up of a plurality of the rotational, vibrational infrared absorption lines associated with the gas of interest are transmitted in the form of a fringe thereby providing a detectable signal from which a determination of the amount of the particular gas of interest can be made. The Fabry-Perot Interferometer which is essential to the operation of this arrangement provides a mirror separation which can be adjusted to simultaneously transmit all of the rotational vibrational infrared absorption lines of a molecular species of the gas of interest. This approach to gas analysis or measurement has provided an advantage in that the sensitivity achieved has been an advance over the existing techniques. However, by relying on a mechanical arrangement for providing the selective separation of the periodic spectra, this approach suffers from certain limitations inherent in the use of a mechanical modulation arrangement. For instance, the accuracy and therefore the sensitivity of this approach is dependent upon the ability to accurately align the mirror elements of the Fabry-Perot Interferometer to the precise bandwidth desired. Additionally inherent in the operation of such mechanical arrangement is the limitation that modifying the operating characteristics of this measurement technique requires a cumbersome and time consuming manual operation involving the actual alignment or tuning of the mirror separation and the verification of the results of this alignment.
Similar to the limitations of the solid electrolyte cell and the AOTF device as applied to a situation requiring the quick, accurate detection or quantification of a gas of interest from a position distant from the environment under study, this use of the etalon device also requires the placement of such device based instrument in the specific area that is to be monitored. Accordingly, this approach also lacks the ability to be operated in a remote survey or scanning mode such that random, removed areas can be tested for the presence or quantity of the gas of interest.